<script>
    var arr = [100,200,300,400]
    console.log(arr)
    console.log(arr.length)

    // 长度的操作
    arr.length = 3
    console.log(arr)
    console.log(arr.length)

    // 数据的操作
    console.log(arr)
    console.log(arr[2])
    arr[2] = 123
    console.log(arr)
    console.log(arr[2])
</script>